Updated actions badges in README

[j0yu]

Closes #785

Updated README.md badges:

• Change badges to only show for master branch runs
• Added/fixed missing Windows workflows
• Added links to workflows pages for each badge
• Added alt text to each badge image

Edits:

1. Removed from PR:

2. Let Improvements on Windows Tests #789 implement

   [![Windows](https://github.com/nerdvegas/rez/workflows/Windows/badge.svg?branch=master)](https://github.com/nerdvegas/rez/actions?query=workflow%3A%22Windows%22+branch%3Amaster)`

[bfloch]

Not sure if we need to expose the Composing badge. It is just a utility workflow that enables the "Windows Docker" workflow and in best case we will only touch it once.

EDIT: More context - it will only trigger if the image related workflows/dockerfiles are touched. It does not add any information to the state of rez passing builds/tests.

[nerdvegas]

I was also going to suggest that we move back to naming the workflow "Windows" rather than "Windows Docker" (use of docker for the test is an implementation detail). I didn't want to make that change at the same time as merging Blazej's PR though.

[j0yu]

Ok, I'll let #789 handle the Windows badge. I've reverted it in this PR so hopefully the merges will be cleaner

I'll leave this snippet here @bfloch for #789

[![Windows](https://github.com/nerdvegas/rez/workflows/Windows/badge.svg?branch=master)](https://github.com/nerdvegas/rez/actions?query=workflow%3A%22Windows%22+branch%3Amaster)`

[nerdvegas]

Merged in https://github.com/nerdvegas/rez/releases/tag/2.47.13